Problem: Which of these numbers is prime? ${21,\ 49,\ 83,\ 90,\ 99}$
Answer: A prime number has exactly two factors: 1 and itself. The factors of 21 are 1, 3, 7, and 21. The factors of 49 are 1, 7, and 49. The factors of 83 are 1 and 83. The factors of 90 are 1, 2, 3, 5, 6, 9, 10, 15, 18, 30, 45, and 90. The factors of 99 are 1, 3, 9, 11, 33, and 99. Thus, 83 is a prime number.